In the northern grain region of Australia, resistance has been confirmed in nine major weed species for the three most important groups of herbicides. Fop (group A), sulfonylurea (group B) and triazine (group C) are among the most commonly used herbicides in wheat and sorghum, and resistance to these herbicides has been reported in several weed species. Implications of these research findings are important in the development and application of weed management strategies.

Common sowthistle or milk thistle (Sonchus oleraceus). 19 chlorsulfuron resistant collections of common sowthistle have been identified. These resistant collections were discovered throughout the northern grain region at various sites around North Star, north (~50km) and north west (~70km) of Goondiwindi, and ~30km south of Meandarra growing in continuously cropped wheat paddocks, where chlorsulfuron had been used for between 4 and 10 years.

Turnip weed (Rapistrum rugosum). Two resistant collections were discovered in 1995 near Boggabilla (Table 1). They were identified from continuously cropped wheat paddocks where chlorsulfuron (Group B) had been used for between 3 and 6 years. Resistance was also confirmed to
metsulfuron-methyl, also a Group B herbicide.

Indian hedge mustard (Sisymbrium orientale). Originally three chlorsulfuron resistant collections were discovered in Goondiwindi and North Star districts. These collections were growing in continuously cropped wheat paddocks where chlorsulfuron had been applied for between 6 and 10 years. A further six collections, from surrounding districts of Goondiwindi, were found to be resistant to chlorsulfuron in later testings.

Climbing buckwheat or black bindweed (Fallopia convolvulus).

One resistant collection was discovered at Bungunya in 1993. It was growing in a continuously cropped wheat paddock where chlorsulfuron had been used for 10 years. This collection was highly resistant to chlorsulfuron in terms of plant survival (>16 fold) and lack of growth reduction, but was not resistant to other herbicide groups (Table 2).

African turnip weed (Sisymbrium thellungii). One resistant collection was discovered in 1995 growing in a continuously cropped wheat paddock, where chlorsulfuron had been used for a minimum of 5 years. This resistant collection from Wallumbilla near Roma was shown to have a greater than 32-fold plant survival when compared to a susceptible collection, but was not resistant to herbicides in other groups. Another collection was also identified as being resistant to chlorsulfuron and
metsulfuron-methyl. Further testing with sulfometuron-methyl, a herbicide also from group B, showed that it did control the resistant collection, although not to the same degree as bromoxymil and 2,4-D.

Liverseed grass (Urochloa panicoides). Seven resistant collections of liverseed grass from sorghum paddocks in eastern Darling Downs were discovered in 1995. They were growing in summer cropped sites where atrazine had been used for between 2 and 7 years. One collection had a high resistance to atrazine with a 16-fold increase in plant survival rate, but was not resistant to other herbicide groups. All of the resistant populations were from the Clifton, Millmerran, Jondaryn and Warwick areas.

Wild oats (Avena sterilis ssp. ludoviciana).
In 1996 one resistant collection was discovered near Croppa Creek, 50 km south of Goondiwindi. It was collected from a paddock with 5 years use of 'fop' and 'dim' herbicides (group A). This collection had a high resistance to clodinafop-propargyl and
fenoxaprop-p-ethyl (both Group A) with >20-fold increase in plant survival. There was no evidence of resistance at that stage to
sethoxydim, which belongs to a different chemical class within Group A. Later testing showed 16 additional wild oat collections resistant to
fenoxaprop. Furthermore, resistance to sethoxydim was found in four out of nine populations tested although three of these collections were controlled by amounts of Sertin higher than the recommended dosage. Several other herbicides also tested, triallate (Group E), glyphosate (Group M) and
flamprop-methyl (Group K) were all found to control the resistant collections.
Clodinafop-propargyl, fenoxaprop-ethyl, sethoxydim, and Tralkoxydim (all group A) exhibited some form of resistance. (See the summary table for these results).

Charlock (Sinapis arvensis).

One population was found to be resistant to chlorsulfuron in NSW in 1996.

Paradoxa grass (Phalaris paradoxa). Two collections of seed from the same property at Croppa Creek, NSW were found to be resistant to
fenoxaprop-ethyl (group A). No confirmed populations of chlorsulfuron resistance have been found.

TO NOTE:

The species listed here are the species we have tested that show herbicide resistance. This list can give an indication of the species most likely to show herbicide resistance in these areas with sustained single herbicide group usage but this does not mean that because you have these weeds in your paddocks they are going to be resistant to your herbicides. Instead you should be aware of the possibility and keep an eye out for any change in growth patterns. Differences in the reproductive biology of the various weed species will ensure that there is variation between species
(ie. not all species will become resistant to the same herbicide). If your paddocks do contain weeds that you suspect are herbicide resistant, check your previous herbicide application history and contact your local agronomist.

The results of the resistant populations showed that resistance had developed at some stage after 3 to 10 years of selection with chlorsulfuron, 5 years 'fop usage, and between 2 to 7yrs atrazine usage.
* Of the four collections of wild oats resistant to Sertin at the recommended dose, only one was resistant at higher
doses.

The most recent collection of broadleaf weed samples (2000) was examined for resistance to chlorsulfuron. It was found that the African turnip populations (2) showed no resistance, while all the populations for the sowthistle (3), climbing buckwheat (1) and Indian hedge mustard (2) were resistant (Table 3).

Recently another 20 populations of wild oats, some collected in 2000, were examined for resistance to
fenoxaprop-ethyl. Out of these collections, 12 were highly resistant with the majority surviving herbicide application rate of 200% of the recommended rate of 400mL/ha. Several of these 12 resistant collections came from different areas within the same property, hence highlighting the magnitude of the resistance problem.

Table 3. Results of testing for herbicide resistance for the 2000 collection

Species

Herbicide

No. samples tested

No. samples resistant

Sow thistle

Glean

3

3

Climbing buckwheat

Glean

1

1

African turnip weed

Glean

2

0

Indian hedge mustard

Glean

2

2

Wild oats

Puma

20

12

Soil samples were collected from two sites around Croppa Creek, NSW where herbicide resistant wild oat and paradoxa grass populations had been detected. The seeds were removed from these samples, grown in pots and sprayed with
fenoxaprop-ethyl at the recommended rate of 400mL/ha, and at twice this rate, to determine the level of resistance present in the seeds in the soil. There was a 100% survival rate of wild oat sprayed seedlings grown from seeds in the soil. The dry weights of plants sprayed with twice the recommended dose of herbicide were 65-80% of the unsprayed plants.

The decline in resistance within the seedbank over time was also determined. It was seen that within 2 years the number of seeds, all of which exhibited resistance, had declined from
200/m2 to 2/m2. This shows that, although a wild oats population may be resistant, the majority of seeds do not persist in the soil seed bank for longer than 2 years. This information suggests that resistant wild oats populations can be managed by changing from winter to summer crops over a two to three year period, and completely preventing any replenishment of the seed
bank.

Figure 2. Decline in wild oats seed numbers over time of which 100% were resistant to Group A herbicides

Several resistant wild oats populations were evaluated further against a variety of herbicides (Figure 3) to test for any possible cross resistance to other herbicide groups. All the fop and dim herbicides (both Group A) exhibited some form of resistance, whereas all herbicides from other groups showed complete control.

Figure 3. Resistance levels of 2 wild oats collections with various herbicides, (C2 was not tested with Achieve®).

Control of herbicide resistant weeds is a great concern. Many of the alternative control methods are often more expensive and inconvenient. In an effort to try and devise appropriate management strategies, a series of practices were observed to evaluate the usefulness of these strategies.

Nine paddocks were selected that had herbicide resistant weeds. Weed densities were recorded over a two-year period in fixed quadrats in early winter and mid spring. The type and effectiveness of the management practices were also evaluated.

Initial observations showed the two most common control methods were combinations of alternating herbicides and changing to summer cropping to allow for a winter fallow. Other practices included growing a lucerne pasture in conjunction with sheep grazing to manage the weeds. Rotation to other herbicide groups but continuing with winter crops had variable results in control of these herbicide resistant weeds. Further information can be gained from the paper by
G.R. Robinson et al (1999) presented at the 12th Australian Weeds Conference in Hobart, Tasmania or by contacting Dr Steve Walker.

Steve Walker gave an invited presentation to the recent GRDC Update Seminars on the topic of 'Glyphosate resistance - should we be doing something?' This paper can be viewed at
www.dpi.qld.gov.au/fieldcrops/3320 on the DPI website.

Andrew Storrie will be making a presentation at the Weed Society of Queensland's symposium 'Technology developments in weed management' on 'The evolution of glyphosate resistance in northern NSW' on 8th August at Dalby.

In USA, populations of horseweed or fleabane (Conyza canadensis) have been found to contain resistance to glyphosate (Group M). Experiments conducted by the University of Delaware confirmed this resistance saying that it is the first broadleaf weed to resist glyphosate. In the 1990's farmers in these areas were encouraged to adopt soybeans genetically engineered for Roundup resistance so that broad-scale spraying of the chemical would control all weeds. It seems that horseweed is currently the only surviving weed in these paddocks.

The known extent of glyphosate resistance worldwide has so far been in the USA, Malaysia and Australia. In America glyphosate resistance has been detected in horseweed
(Conzya canadensis) and ryegrass (Lolium rigidium), while in Malaysia crowsfoot grass
(Eleusine indica) was found to be resistant. In Australia, resistance has been found in several populations across southern Australia in annual ryegrass (Lolium
rigidium).

In the northern grain region, there is also glyphosate resistance found on the Liverpool Plains near Tamworth, where wheat / sorghum rotations are used and have a history of glyphosate usage. The spread of glyphosate resistance in the northern grain region will depend on the rate and intensity at which the glyphosate is applied. Andrew
Storrie, Weeds Agronomist with NSW Agriculture in Tamworth, was responsible for the detection of these resistant populations and can be contacted for more information on (02) 6763 1174 (phone).

There has been no evidence of glyphosate resistance in any of the weed species we tested in the northern grains region.

Testing for herbicide resistance usually takes several months if done by pot trials. More rapid techniques therefore would be advantageous in recognizing resistance so that management practices could be undertaken as soon as possible.

The Novartis Biology Department Headquarters in Switzerland developed a "quick-test" method for detecting herbicide resistance that used cuttings rather than plants obtained from seeds in the detection. This method has some advantages in that the plants are detected for herbicide resistance during the growing season and hence the results can be obtained before the seed is shed into the soil seedbank. Problems with this test are also foreseeable especially in plants not able to be grown from cuttings.

Other alternative methods for rapid testing of herbicide resistance are being examined at the University of Queensland. Molecular techniques are being evaluated for the potential development of a diagnostic screening test for herbicide resistance. Recent results are looking promising and we shall keep you posted.

A very good website has been setup for internationally recognized herbicide resistant weeds. It provides information on the weeds, the herbicides and gives specific information on the degree of resistance found across the globe. Many of the resistant populations referred to in this newsletter have been registered on this site by Steve Walker. The address of this site is
www.weedscience.org

Other websites of interest include the Western Australia Herbicide Resistance Initiative
(WAHRI) www.wahri.agric.uwa.edu.au and the Western Australian Agriculture Weeds info
www.agric.wa.gov.au/cropupdates/1999/weeds/index to name just two. In Queensland we have several articles of interest found under the DNR webpage
(www.dnr.qld.gov.au). We, at the
TSWRU, are also in the process of developing a webpage www.weeds.uq.edu.au so keep your eyes out for this one also.

With the end of this project drawing near, we would like to thank all those farmers and industry representatives who have been involved in this project in some way. We appreciate the cooperation in seed collection and the interest and patience in waiting for the results.

We wish you all the very best in your enterprises and encourage you to be on the lookout for herbicide resistance in your weeds. Should you suspect resistance, contact your local agronomist or chemical industry supplier.

Further funding from the GRDC has been granted to undertake two new herbicide resistance projects. Both projects involve collaboration between University, Government and Industry sectors with community involvement. One project is facilitated through the University of Queensland and led by Dr Steve Adkins entitled the "Detection, Monitoring and Management of Herbicide Resistance."

The other project, facilitated through the DPI Toowoomba, is entitled "Risk assessment and preventative IWM strategies for herbicide resistance in the diverse farming systems in the Northern Region". This project involves a team of 11 weed scientists and technicians from
QDPI, NSW Agriculture and UQ, and lead by Dr Steve Walker.

Further editions of the Herbicide Reporter will be produced as a result of both projects to keep you informed of progress. We shall also endeavour to report to you on other resistance work happening around Australia.

We would be happy to provide further feedback on any aspect within this reporter should you be interested. We envisage future testing for herbicide resistance in weeds. The TSWRU, Tropical and Subtropical Weed Research Unit, is currently developing a rapid diagnostic testing service for people in the northern grains regions. We will notify people when this service becomes available.